CN115453265A - Island microgrid fault transient protection method based on fault initial-stage current waveform characteristics - Google Patents
Island microgrid fault transient protection method based on fault initial-stage current waveform characteristics Download PDFInfo
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Abstract
The method comprises the steps of sampling currents on two sides of a protected line in an island micro-grid system, calculating the difference value of the current time currents on the two sides and the current time corresponding to the previous power frequency period and the sum of the first-order change gradient of the current time currents, judging whether the micro-grid system fails or not, if the micro-grid system fails, acquiring current data of the initial stage of the failure, sequentially calculating the expansion corrosion differential value, the skewness coefficient and the energy value of the current of the initial stage of the failure, judging whether the product of the skewness coefficients of the expansion corrosion differential values of the current of the initial stage of the failure on the two sides is larger than zero or not, if the fault data is judged, judging that the line is a fault line with the micro-source connected on the two sides and the phase j is a fault phase, and if the difference between the current energy values of the initial stages of the failure on the two sides is larger than the energy threshold value, judging that the line is a fault line with the micro-source connected on one side and the phase is a fault phase, and otherwise judging that the line is a non-fault line or a non-fault line with the fault line.
Description
Technical Field
The invention relates to the technical field of power systems, in particular to a fault transient protection method for an island microgrid based on current waveform characteristics at the initial stage of a fault.
Background
As an effective mode and way for solving the difficult problem of reliable power supply of off-land islands, the island micro-grid has the capacity of new energy consumption and independent operation, is far away from the center of land power supply, and supplies power to loads only by distributed micro-sources such as diesel generation, wind power, photovoltaic and the like in the micro-grid. The island microgrid determined by a special geographical position may face severe environmental challenges such as lightning storm, low temperature and the like at any time, so that short-circuit faults of the system frequently occur. However, because the island microgrid lacks large grid support, the island microgrid has the characteristics of weak system inertia, small damping, poor overcurrent capacity of a power electronic converter and the like, and the island microgrid has poor short-circuit fault tolerance capability, so that the problems of burning out of important equipment and even system instability are easily caused, and therefore, the research on a rapid and reliable island microgrid short-circuit fault protection technology is urgently needed.
After the island microgrid has a short-circuit fault, inverter type micro sources such as wind power and photovoltaic power can start corresponding fault ride-through control strategies, the output current of the inverter is limited within 1.5-2 times of rated current, the switching influence of the inverter control strategies is considered, the fault transient response process of the inverter type micro sources contains abundant fault information and generally occurs within 5ms after the fault, and the fault transient information introduced by the inverter type micro sources can be used for realizing rapid fault detection. The existing microgrid protection method is mostly based on fault steady-state information such as current amplitude, phase and power direction, usually at least one power frequency cycle is needed to complete fault detection and judgment, and the control of the inverter output current amplitude and phase by a fault ride-through control strategy can cause the difference of fault steady-state characteristics inside and outside a microgrid system area to be small, so that the requirements of protection selectivity and rapidity of the microgrid system are difficult to meet. Therefore, the invention constructs a protection criterion by using the transient state characteristics of the initial fault current, and provides a transient state protection method for the fault of the island microgrid based on the waveform characteristics of the initial fault current.
Disclosure of Invention
The invention aims to provide an island microgrid fault transient protection method based on fault initial-stage current waveform characteristics, which can quickly judge fault lines and fault phases.
In order to solve the technical problems, the invention adopts the following technical method: an island microgrid fault transient protection method based on fault initial-stage current waveform characteristics comprises the following steps:
step S1, two sides of a protected line in the island microgrid system are respectively defined as an e side and an f side, the protection devices on the e side and the f side respectively collect current signals on one side of the protected line in real time, store current data in a power frequency period and generate a current sampling value sequence i ej (k) And i fj (k) J represents phases a, b and c, and k represents the kth sampling value;
s2, calculating the difference value delta i between the current j phase current at the current moment of the e side of the protected line and the current j phase current at the corresponding moment of the previous power frequency period ej (k) And the difference value delta i between the current j phase current at the current moment of the f side and the current j phase current at the moment corresponding to the previous power frequency cycle fj (k)；
Step S3, calculating the current difference value delta i obtained in the step S2 by adopting the following formula (1) ej (k) First order gradient sum of K ej (k) And the current difference value Deltai fj (k) First order gradient sum of K fj (k) Judging the first order change gradient sum K ej (k)、K fj (k) Whether all are greater than a threshold value K set If not, turning to the step S1; if yes, the island microgrid systemThe e side and f side protection devices of the medium protected line mark the current time as the fault time, and collect the current data of the e side and f side of the protected line in x power frequency periods after the fault;
in the formula, Q represents the number of sampling points;
s4, firstly, calculating the expansion operation difference value delta f of the current on the e side of the protected line in x power frequency periods after the fault by using mathematical morphological expansion and corrosion operation and a formula (3) dile_j (n) and the differential value Δ f of the corrosion operation eroe_j (n) and the expansion difference value Deltaf of the f-side current dilf_j (n) and the differential value Δ f of the etching operation erof_j (n), the definition formula of the mathematical morphology expansion and corrosion operation is as the following formula (2), and the expansion corrosion operation difference value f of the protected line e side current in x power frequency periods after the fault is obtained according to the formula (4) grade_j (n) and the differential value f calculated by the dilation-erosion of the current on the f-side gradf_j (n), finally, calculating the differential value f of the expansion corrosion operation according to the formula (5) grade_j Skewness coefficient S of (n) fgrade_j And computing the differential value f by dilation-erosion gradf_j Skewness coefficient S of (n) fgradf_j ；
Wherein f (n) represents an input signal, 0. Ltoreq.n<N, N is input signal length, g (m) represents structural element, 0 ≦ m<M, M is the length of the structural element, N is greater than M, symbol
Representing a mathematical morphological dilation operation,
representing mathematical morphological erosion operations;
Δf dil (n)＝f dil (n)-f dil (n-1)；Δf ero (n)＝f ero (n)-f ero (n-1) (3)
in the formula, mu is the differential value f of the expansion corrosion operation grad (n) the average value of (n),
sigma is differential value f of expansion corrosion operation grad (n) the standard deviation of the (n),
s5, calculating the energy values of currents on the e side and the f side of the protected line in x power frequency periods after the fault according to the following formula (6);
in the formula, E ej And E fj Respectively representing the energy value of j phase current at the e side of the protected line and the energy value of j phase current at the f side of the protected line in x power frequency periods after the fault;
s6, starting a protection criterion # 1, and judging the differential value f of the expansion corrosion operation of the e-side current of the protected line within 5ms after the fault grade_j Skewness coefficient S of (n) fgrade_j And the differential value f of the expansion corrosion operation of the f-side current gradf_j Skewness coefficient S of (n) fgradf_j If the product of the positive phase and the negative phase is greater than zero, judging that the protected line is a fault line with both sides connected with inverter type micro sources and the j phase is a fault phase; if the current is less than zero, starting a protection criterion # 2, and judging the difference | E between the energy value of the E-side current and the energy value of the f-side current of the protected line within 5ms after the fault ej -E fj Whether | is much greater than energy threshold E set If yes, judging that the protected line is a fault line with one side connected with an inverter type micro source and j phase is a fault phase; if not, the protected line is judged to be a non-fault line or a non-fault phase of the fault line.
Further, in step S1, sampling frequencies of protection devices on the e side and the f side of a protected line in the island microgrid system are 10kHz, that is, 200 sampling points are collected within 20ms of a power frequency cycle.
Furthermore, the x power frequency periods are quarter of one power frequency period, namely 5ms.
Further, in step S3, the current difference Δ i is calculated using equation (1) ej (k) First order gradient sum of K ej (k) And the current difference value Deltai fj (k) First order gradient sum of K fj (k) The number of sampling points Q is 10.
Still further, the threshold value K in step S3 set Is 0.01.
Preferably, the energy threshold E in step S6 set The current energy at two sides of the protected line is the minimum within 5ms after the fault10 times the value.
The invention provides a fault transient protection method for an island microgrid based on fault initial-stage current waveform characteristics, which is characterized in that each inverter type micro-source outputs fault current with phase jump at the initial stage of fault and the fault current flows through each feeder line to be fed into a fault point, and the phase jump directions of the fault initial-stage current at both sides of a fault line and a non-fault line are different. According to the method, the current phase jump direction at the initial stage of the fault is represented by the skewness coefficient of the mathematical morphology expansion corrosion operation differential value, and the current amplitude condition at the initial stage of the fault is represented by the current energy value at the initial stage of the fault, so that when the island microgrid has a short-circuit fault, the protection devices at two sides of the protected line can realize the judgment of the fault line and the fault phase. The method only needs to exchange the skewness coefficient of the fault initial-stage current expansion corrosion operation differential value and the fault initial-stage current energy value at two sides of the protected line, the communication data volume is small, and the requirements on the communication bandwidth and the speed are low. And after the micro-grid system breaks down, the inverter type micro-source starts a current amplitude limiting control strategy for current suppression within 5ms after the fault occurs, namely the output current of the inverter type micro-source generates a phase jump phenomenon within 5ms after the fault occurs, so that the fault line and the fault phase can be rapidly judged after the fault occurs, and then the protection devices on the two sides of the fault line can rapidly cut off the circuit breakers of the fault phase of the fault line, so that the fault transient protection is realized.
Drawings
FIG. 1 is a diagram of a typical island microgrid system;
FIG. 2 is a current waveform on two sides of a line at the initial stage of a fault in an island microgrid; (a) When both sides are connected with inverter type micro-sources, the fault phase current waveforms on both sides of the fault line are generated; (b) When only one side is connected with an inverter type micro source, the fault phase current waveforms on two sides of a fault line are generated; (c) current waveforms on both sides of the non-fault line;
FIG. 3 is the differential value of the expansive corrosion operation of the primary current of the fault in the island microgrid; (a) When both sides are connected with inverter type micro-sources, the differential value of the expansion corrosion operation of the fault phase current waveforms on both sides of the fault line is calculated; (b) When only one side is connected with an inverter type micro source, the differential value of the expansion corrosion operation of the fault phase current waveforms on the two sides of the fault line is calculated; (c) The differential value of the expansion corrosion operation of the current waveforms on the two sides of the non-fault line is calculated;
fig. 4 is a flow chart of an island microgrid fault transient protection method based on fault primary current waveform characteristics.
Detailed Description
In order to facilitate understanding of those skilled in the art, the present invention is further described below with reference to the following examples and the accompanying drawings, which are not intended to limit the present invention.
As shown in fig. 1, a typical island microgrid system structure mainly includes components such as a photovoltaic power generation unit, a wind power generation unit, an energy storage system, a power load, a current measurement unit, a circuit breaker, and the like, a photovoltaic inverter adopts a PQ control strategy, an energy storage inverter adopts a constant-frequency constant-voltage control strategy (Vf control), these inverter type micro sources are all connected to a 10kV network through an interface transformer, the low-voltage side of the interface transformer is in a Y-type winding connection mode, so that power is supplied to a single-phase load, and the high-voltage side is in a delta-type winding connection mode.
For the photovoltaic inverter adopting the PQ control strategy, the photovoltaic inverter is in a unit power factor working state during normal operation, and the phase difference between the output voltage and the current of the photovoltaic inverter is 0 degree. When a short-circuit fault occurs on a certain feeder line in a microgrid, the voltage of a system bus drops, when the instantaneous value of any phase current is greater than the maximum allowable value (generally 1.2p.u. -2p.u.) of a photovoltaic inverter, the photovoltaic inverter starts a current amplitude limiting control strategy, the reactive current and the active current reference value of a current control loop are adjusted to be reference values shown in formulas (7) - (8), and the phase difference between the output voltage and the current of the photovoltaic inverter is (0 DEG, 90 DEG), so that the response speed of the output current of the photovoltaic inverter changing along with the reference current is considered to be fast, and the output current waveform can be approximately considered to have one phase jump at the moment of starting the current amplitude limiting control.
In the formula i * Ld And i * Lq Active and reactive current reference values, I, adjusted for current-limiting control strategies max Is the maximum allowable value, V of Outputting positive sequence voltage amplitude, V, for the photovoltaic inverter after failure n Is a rated voltage value.
For the energy storage inverter adopting the Vf control strategy, the inverter is in a rated operation state during normal operation, and the output voltage of the energy storage inverter can track a voltage reference value with zero error and can be equivalent to a constant voltage source with zero internal impedance. Because the resistance inductance of the 10kV microgrid circuit is large, the phase difference of the output voltage and the current of the energy storage inverter can be approximately considered to be equal to 0 degree. When a short-circuit fault occurs on a certain feeder line in a microgrid, the voltage of a system bus drops, when the instantaneous value of any phase current is greater than the maximum allowable value (generally 1.2p.u. -2p.u.) of the energy storage inverter, the energy storage inverter can start a current amplitude limiting control strategy, the inverter is switched from a Vf control mode to a current control mode, the reactive current and the active current reference value of a current control loop are also shown in formulas (7) - (8), the phase difference of the output voltage and current of the energy storage inverter is between (0 degrees and 90 degrees), and similarly, the response speed of the output current of the energy storage inverter changing along with the reference current is considered to be very fast, and the output current waveform can be approximately considered to have undergone a phase jump at the moment of starting the current amplitude limiting control.
After the island micro-grid system has short-circuit fault, each inverter type micro-source outputs fault current with phase jump at the initial stage of the fault, and the fault current flows through each feeder line and is fed into a fault point. For the fault line with both sides of the line connected with the inverter type micro-source, the fault phase current directions at both sides of the line are the same, namely the phase jump directions are the same; for a fault line only one side of which is connected with an inverter type micro-source, the phase jump directions of fault phase currents on two sides of the line are opposite, and the current on one side connected with the inverter type micro-source is far larger than the current on the other side; the non-fault phase current on the two sides of the fault line and the current on the two sides of the non-fault line have opposite phase jump directions, and the current amplitudes on the two sides of the line are equal. Fault phase current waveforms on two sides of a fault line with inverter type micro sources connected to two sides of a line in the island microgrid, fault phase current waveforms on two sides of a fault line with inverter type micro sources connected to only one side of the line, and current waveforms on two sides of a non-fault line are respectively shown in fig. 2 (a) - (c).
The invention provides an island microgrid transient state protection method based on fault initial-stage current waveform characteristics by utilizing the phase jump characteristic difference of the current waveforms of the fault initial-stage fault line and the non-fault line, the process is shown in fig. 4, and the method comprises the following specific steps:
step S1, two sides of a protected line in an island microgrid system are respectively defined as an e side and an f side, and protection devices on the e side and the f side (because of the characteristics of the microgrid system such as topological structure and bidirectional trend direction, the protection devices are arranged on two sides of the line in the existing microgrid system) respectively collect current signals on one side of the protected line in real time through a current measurement unit, store current data in a power frequency period and generate a current sampling value sequence i ej (k) And i fj (k) J represents phases a, b and c, k represents the kth sampling value, and the sampling frequency is 10kHz, namely 200 sampling points are acquired within 20ms of one power frequency period.
S2, calculating the difference value delta i between the current j phase current at the current moment of the e side of the protected line and the current j phase current at the corresponding moment of the previous power frequency period ej (k) And the difference value delta i between the current j phase current at the current moment of the f side and the current j phase current at the moment corresponding to the previous power frequency cycle fj (k)。
Step S3, calculating the current difference value delta i obtained in the step S2 by adopting the following formula (1) ej (k) First order gradient sum of K ej (k) And the current difference value Δ i fj (k) First order gradient sum of K fj (k) Judging the first order change gradient sum K ej (k)、K fj (k) Whether all are greater than a threshold value K set If not, then go to step S1; if yes, the current time is marked as the fault time by the protective devices on the e side and the f side of the protected line in the island microgrid system, and current data of the e side and the f side of the protected line within 5ms after the fault is collected. Current difference value delta i in normal operation state ej (k)、Δi fj (k) Are all zeroGradient of one order of change and K ej (k)、K fj (k) Also equal to zero, micro grid system failure instant, Δ i ej (k) And Δ i fj (k) Occurrence of sudden increase, K ej (k)、K fj (k) Greater than zero, so that the aforementioned threshold value K set Set to 0.01 to avoid the effects of measurement errors.
In the formula, Q represents the number of sampling points and is set to 10.
Step S4, firstly, calculating the expansion operation differential value delta f of the current on the e side of the protected line within 5ms after the fault by using mathematical morphology expansion and corrosion operation and a formula (3) dile_j (n) and the differential value Δ f of the corrosion operation eroe_j (n) and the expansion difference value Deltaf of the f-side current dilf_j (n) and the differential value Δ f of the corrosion operation erof_j (n) the definition formula of mathematical morphology expansion and corrosion operation is as the following formula (2), and then the expansion corrosion operation difference value f of the protected line e side current within 5ms after the fault is obtained according to the formula (4) grade_j (n) and the differential value f calculated by the dilation-erosion of the current on the f-side gradf_j (n), finally calculating the differential value f of the expansion corrosion operation according to the formula (5) grade_j Skewness coefficient S of (n) fgrade_j And computing the differential value f by dilation-erosion gradf_j Skewness coefficient S of (n) fgradf_j 。
Wherein f (n) represents an input signal, 0. Ltoreq. N<N, N is the input signal length, here 50 sample points, g (m) represents a structural element, 0 ≦ m<M, M is the length of the structural element, N is greater than M, symbol
Represents a mathematical morphological dilation operation and theta represents a mathematical morphological erosion operation.
Δf dil (n)＝f dil (n)-f dil (n-1)；Δf ero (n)＝f ero (n)-f ero (n-1) (3)
Where μ is the difference f of the expansion corrosion operation grad (n) the average value of (n),
sigma is differential value f of expansion corrosion operation grad (n) the standard deviation of the (n),
since the expansion and erosion operations are respectively the process of expanding the boundary of the input signal to the outside and contracting the boundary to the inside, the expansion erosion differential operation of the input signal can be used for edge detection, which is defined as formula (4), when Δ f is dil (n) and Δ f ero When (n) is greater than zero, the differential value f of the expansion corrosion operation of the input signal grad (n) is equal to Δ f dil (n) and Δ f ero Large minus small in (n), f grad (n) is a positive number, the input signal is considered to be on a rising edge; when Δ f dil (n) and Δ f ero (n) are all less than or equal to zero, the differential value f of the expansion corrosion operation of the input signal grad (n) is equal to Δ f dil (n) and Δ f ero Small value minus large value in (n), f grad (n) is a negative number, and the input signal is considered to be on a falling edge. As shown in fig. 3 (a) - (c), for the fault line with the inverter type micro source connected to both sides of the line, the phase jump direction of the fault phase current on both sides of the line is the same, and the expansion of the fault phase current waveform on both sides of the line is the sameThe differential value distribution of the bulging corrosion operation is biased to a negative half shaft; for a fault line only one side of the line is connected with an inverter type micro source, the phase jump directions of fault phase currents on two sides of the line are opposite, the expansion corrosion operation differential values of the fault phase current waveforms on two sides of the line are opposite in distribution, one side of the fault line is deviated to a positive half shaft, and the other side of the fault line is deviated to a negative half shaft; for a non-fault line, both sides of the non-fault line are connected with the micro source or only one side of the non-fault line is connected with the micro source, the current phase jump directions of both sides of the line are opposite, the expansion corrosion operation differential value distribution of the current waveform of one side of the line is biased to a positive half shaft, and the other side of the line is biased to a negative half shaft. Therefore, we can know that the distribution situation of the expansion corrosion operation differential value of the current can be used for judging the fault line and the fault phase of the micro-source connected to the two sides of the line, and the calculation skewness coefficient can convert the distribution situation of the positive half shaft and the negative half shaft of the expansion corrosion operation differential value (a group of data) into a numerical value, the numerical value distribution is deviated towards the positive half shaft, the skewness coefficient is larger than zero, the numerical value distribution is deviated towards the negative half shaft, and the skewness coefficient is smaller than zero, so that the communication data quantity on the two sides of the line can be reduced, and the communication bandwidth requirement is reduced. According to the description and analysis of fig. 2 (a) - (c) and fig. 3 (a) - (c), we can not distinguish "the fault line and the fault phase connected with the micro source on only one side of the line" from "the non-fault line connected with the micro source on both sides or connected with the micro source on only one side" according to the expansion corrosion operation differential value distribution of the current, however, as shown in fig. 2 (a) - (c), for the fault line connected with the inverter type micro source on only one side of the line, the phase jump direction of the fault phase current on both sides of the line is opposite, and the current on one side connected with the inverter type micro source is much larger than the current on the other side; the non-fault phase current on the two sides of the fault line is opposite to the phase jump direction of the current on the two sides of the non-fault line, and the current amplitudes on the two sides of the line are equal. Accordingly, we can know that the two conditions can be effectively partitioned by combining the skewness coefficient of the differential value and the energy value of the current through the expansion corrosion operation of the current, and then the skewness coefficient and the energy value of the current are introducedStep S5 is provided, which provides the basis for constructing the protection criterion # 2.
And step S5, calculating the energy values of the e-side current and the f-side current of the protected line within 5ms after the fault according to the following formula (6).
In the formula, E ej And E fj The energy values of the j-side phase current of the protected line e and the j-side phase current of the protected line f within 5ms after the fault are respectively shown.
And (5) integrating the steps S4 and S5, and pre-constructing a protection criterion # 1 and a protection criterion # 2 for realizing the discrimination of the fault line and the fault phase of the island microgrid.
Protection criterion #1: judging the differential value f of the expansion corrosion operation of the protected line e side current within 5ms after the fault grade_j Skewness coefficient S of (n) fgrade_j And the differential value f of the expansion corrosion operation of the f-side current gradf_j Skewness coefficient S of (n) fgradf_j If the product of the j phases is greater than zero, the protected line is judged to be a fault line and the j phase is a fault phase; and if the value is less than zero, starting a protection criterion # 2.
Protection criterion #2: judging the difference | E between the energy value of the E side current and the energy value of the f side current of the protected line within 5ms after the fault ej -E fj Whether | is much greater than energy threshold E set If yes, the protected line is judged to be a fault line and the j phase is a fault phase; if not, the protected line is judged to be a non-fault line or a non-fault phase of the fault line.
S6, starting a protection criterion # 1, and judging the differential value f of the expansion corrosion operation of the e-side current of the protected line within 5ms after the fault grade_j Skewness coefficient S of (n) fgrade_j And the differential value f of the expansion corrosion operation of the f-side current gradf_j Skewness coefficient S of (n) fgradf_j If the product of the positive phase and the negative phase is greater than zero, judging that the protected line is a fault line with both sides connected with inverter type micro sources and the j phase is a fault phase; if the value is less than zero, the protection criterion # 2 is startedAnd judging the difference | E between the energy value of the E side current and the energy value of the f side current of the protected line within 5ms after the fault ej -E fj Whether | is much greater than energy threshold E set If yes, judging that the protected line is a fault line with one side connected with an inverter type micro source and the j phase is a fault phase; if not, the protected line is judged to be a non-fault line or a non-fault phase of the fault line. It is noted that the energy threshold E is used here to avoid the effects of measurement errors, sampling synchronization errors, and filter-induced errors set The value is 10 times of the minimum value of the initial fault current energy on two sides of the line.
Step 7, starting protective measures by the protective devices on the e side and the f side of the protected line in the island microgrid system according to the judgment result of the step 6, wherein the protective measures are specifically as follows: if the protection criterion #1 is met in the step S6, the protected line is judged to be a fault line with both sides connected with inverter type micro sources and the j phase is a fault phase, the protection devices on the e side and the f side of the protection line can respectively send tripping signals to the j phase circuit breaker on one side of the protection line, and split-phase tripping is realized; if the protection criterion #1 is not met but the protection criterion #2 is met in the step S6, the protected line is judged to be a fault line with one side connected with an inverter type micro source and the j phase is a fault phase, the protection devices on the e side and the f side of the protection line respectively send tripping signals to the j phase circuit breaker on one side of the protection line, and split-phase tripping is achieved; if the protection criteria #1 and #2 in the step S6 are not satisfied, the protected line is determined to be a non-fault line or a non-fault phase of a fault line, and the protection device of the protected line does not send a trip signal to the j-phase circuit breaker of the protected line.
In summary, the protection devices on both sides of the protected line only need to exchange the skewness coefficient of the expansion corrosion operation differential value of the initial fault current and the energy value of the initial fault current, the communication data volume is small, and the requirements on the communication bandwidth and the speed are low. In addition, after the micro-grid system fails, the inverter type micro-source generally starts a current amplitude limiting control strategy for current suppression within 5ms after the failure, namely the output current of the inverter type micro-source generates a phase jump phenomenon within 5ms after the failure, so that the fault line and the fault phase can be judged within 5ms after the failure.
In order to verify the effectiveness of the invention, a 10kV island micro-grid system model shown in fig. 1 is established in PSCAD/EMTDC, the capacities of inverter type micro-source photovoltaic 1, photovoltaic 2 and a fan are all 500kW, the inverters adopt a PQ control strategy, the energy storage capacity is 500kVA, and the inverters adopt a Vf control strategy. The capacity of load 1 is (1 + j0.03) MVA, and the capacities of load 2 and load 3 are both (0.5 + j0.015) MVA. All the feeder lines have the length of 2km, and the positive sequence impedance and the zero sequence impedance of the unit length of the line are respectively 0.265+ j0.078 omega/km and 2.7+ j0.318 omega/km.
Fig. 1 shows the morphological characteristic values and energy values of the current and the discrimination results of the fault line and the fault phase in 5ms after the fault occurs at the F1 position in the microgrid and the fault resistance is 1 Ω, respectively. As can be seen from the table, the expansion corrosion operation difference value skewness coefficient of the phase A and the phase B currents on the two sides of the feeder line 23 meets the protection criterion # 1, the energy values of the phase A and the phase B currents meet the protection criterion # 2, the expansion corrosion operation difference value skewness coefficient of the currents on the two sides of other feeder lines does not meet the protection criterion # 1, and the energy values of the currents on the two sides do not meet the protection criterion # 2, which indicates that the method can accurately judge that the feeder line 23 is a fault line, the phase A and the phase B are fault phases, and the phase C is a non-fault phase.
TABLE 1
The above embodiments are preferred implementations of the present invention, and besides, the present invention can be implemented in other ways, and any obvious substitutions without departing from the concept of the present invention are within the protection scope of the present invention.
Some of the drawings and descriptions of the present invention have been simplified to facilitate the understanding of the improvements over the prior art by those skilled in the art, and other elements have been omitted from this document for the sake of clarity, and it should be appreciated by those skilled in the art that such omitted elements may also constitute the subject matter of the present invention.
Claims (6)
1. The island microgrid fault transient protection method based on the fault initial-stage current waveform characteristics is characterized by comprising the following steps of:
step S1, two sides of a protected line in the island microgrid system are respectively defined as an e side and an f side, the protection devices on the e side and the f side respectively collect current signals on one side of the protected line in real time, store current data in a power frequency period and generate a current sampling value sequence i ej (k) And i fj (k) J represents phases a, b and c, and k represents the kth sampling value;
s2, calculating the difference value delta i between the current j phase current at the current moment of the e side of the protected line and the current j phase current at the corresponding moment of the previous power frequency period ej (k) And the difference value delta i between the current j phase current at the current moment of the f side and the current j phase current at the moment corresponding to the previous power frequency cycle fj (k)；
Step S3, calculating the current difference value delta i obtained in the step S2 by adopting the following formula (1) ej (k) First order gradient sum of K ej (k) And the current difference value Δ i fj (k) First order gradient sum of K fj (k) Judging the first order change gradient sum K ej (k)、K fj (k) Whether all are greater than a threshold value K set If not, turning to the step S1; if yes, marking the current time as the fault time by the protection devices on the e side and the f side of the protected line in the island microgrid system, and collecting current data of the e side and the f side of the protected line in x power frequency periods after the fault;
in the formula, Q represents the number of sampling points;
s4, firstly, calculating the expansion operation difference value delta f of the current on the e side of the protected line in x power frequency periods after the fault by using mathematical morphological expansion and corrosion operation and a formula (3) dile_j (n) and the differential value Δ f of the etching operation eroe_j (n) and the expansion operation differential value Deltaf of the f-side current dilf_j (n) and the differential value Δ f of the etching operation erof_j (n) the mathematics ofThe definition formula of the morphological dilation and erosion operation is as the following formula (2), and the dilation erosion operation difference value f of the protected line e side current in x power frequency periods after the fault is obtained according to the formula (4) grade_j (n) and the differential value f calculated by the dilation-erosion of the current on the f-side gradf_j (n), finally calculating the differential value f of the expansion corrosion operation according to the formula (5) grade_j Skewness coefficient S of (n) fgrade_j And computing the differential value f by dilation-erosion gradf_j Skewness coefficient S of (n) fgradf_j ；
Wherein f (n) represents an input signal, 0. Ltoreq. N<N, N is the length of the input signal, g (m) represents a structural element, 0 ≦ m<M, M is the length of the structural element, N is greater than M, symbol
The mathematical morphological dilation operation is represented as a mathematical morphological dilation operation,
representing mathematical morphological erosion operations;
Δf dil (n)＝f dil (n)-f dil (n-1)；Δf ero (n)＝f ero (n)-f ero (n-1) (3)
in the formula, mu is the differential value f of the expansion corrosion operation grad (n) the average value of the (n),
sigma is differential value f of expansion corrosion operation grad (n) the standard deviation of the (n),
s5, calculating the energy values of currents on the e side and the f side of the protected line in x power frequency periods after the fault according to the following formula (6);
in the formula, E ej And E fj Respectively representing the energy value of j phase current at the e side of the protected line and the energy value of j phase current at the f side of the protected line in x power frequency periods after the fault;
s6, starting a protection criterion #1, and judging the differential value f of the expansion corrosion operation of the e-side current of the protected line within 5ms after the fault grade_j Skewness coefficient S of (n) fgrade_j And the differential value f of the swelling corrosion operation of the f-side current gradf_j Skewness coefficient S of (n) fgradf_j If the product of the positive phase and the negative phase is greater than zero, judging that the protected line is a fault line with both sides connected with inverter type micro sources and the j phase is a fault phase; if the current is less than zero, starting a protection criterion #2, and judging the difference | E between the energy value of the E-side current and the energy value of the f-side current of the protected line within 5ms after the fault ej -E fj Whether | is much greater than the energy threshold E set If yes, judging that the protected line is a fault line with one side connected with an inverter type micro source and the j phase is a fault phase; if not, judging that the protected line is a non-fault line or a non-fault phase of a fault line;
step 7, starting protective measures by the protective devices on the e side and the f side of the protected lines in the island microgrid system according to the judgment result of the step 6, wherein the protective measures are specifically as follows: if the protected line is judged to be a fault line with both sides connected with the inverter type micro source and the j phase is a fault phase in the step S6, the protection devices on the e side and the f side of the protection line respectively send tripping signals to the j phase circuit breaker on one side of the protection line, so that split-phase tripping is realized; if the protected line is judged to be a fault line with one side connected with an inverter type micro source and the j phase is a fault phase in the step S6, the protection devices on the e side and the f side of the protection line respectively send tripping signals to the j-phase circuit breaker on one side of the protection line, so that split-phase tripping is realized; if it is determined in step S6 that the line to be protected is a non-faulty line or a non-faulty phase of a faulty line, the protection device of the line to be protected does not send a trip signal to its j-phase circuit breaker.
2. The island microgrid fault transient protection method based on primary fault current waveform characteristics of claim 1, characterized in that: in step S1, sampling frequencies of protection devices on e-side and f-side of a protected line in the island microgrid system are 10kHz, that is, 200 sampling points are collected within 20ms of a power frequency cycle.
3. The island microgrid fault transient protection method based on primary fault current waveform characteristics of claim 2, characterized in that: the x power frequency periods are one quarter of one power frequency period, namely 5ms.
4. The island microgrid fault transient protection method based on primary fault current waveform characteristics of claim 3, characterized in that: in step S3, the current difference value Δ i is calculated using the formula (1) ej (k) First order gradient sum of K ej (k) And the current difference value Δ i fj (k) First order gradient sum of K fj (k) The number of sampling points Q is 10.
5. The island microgrid fault transient protection method based on primary fault current waveform characteristics of claim 4, characterized in that: the threshold value K in step S3 set Is 0.01.
6. The island microgrid fault transient protection method based on the primary fault current waveform characteristics of claim 5, characterized in that: the energy threshold E in step S6 set The current energy of two sides of the protected line within 5ms after the fault10 times the minimum value of the quantity.
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